Might having a urinary tract infection cause an elderly person to lose his ability to speak and interact?

No. Not unless that infection spreads, becomes very severe and affects the parts of the brain controlling speech. This would be incredibly rare.
Highly unlikely. There is no reason to lose ability to speak unless this is a rare side effect from medications taken for the uti.

Related Questions

What are the best places to assess urinary tract infection in the elderly?

Dr's office/clinic. Urologists are experienced in working up patients with utis and can perform office cystoscopies (telescopic examination of bladder & urethra) geriatricians are experienced in taking care of the elderly. Interested internists & family practioners can also do a good job & refer to a urologist if necessary. Not an er. Read more...